1. Field of the Invention
The invention relates to aqueous coating compositions based on polyurethane dispersions which cure under the influence of high-energy radiation, to a process for preparing them and to their use.
2. Description of the Related Art
Known from the prior art are aqueous one-component (1K) coating compositions which comprise a binder containing active hydrogen atoms and a crosslinker and which are crosslinked and cured by thermal activation. A disadvantage of these systems, however, is that curing is slow and requires high temperatures.
Likewise known are aqueous coating compositions which cure physically, corresponding to a curing of the coat by filming. In systems of this kind, normally no crosslinking agents are used. Physical curing can be assisted by chemical crosslinking by means of atmospheric oxygen.
Substantially more rapid is the curing of coatings based on radiation-curable polyurethane dispersions: that is, polyurethane dispersions containing polymerizable groups. Such dispersions are described in, for example, EP-A 0 942 022. A disadvantage there is that the radiation-curable coatings only cure completely when they are irradiated with a certain dose and intensity, for example with UV light. If it is desired to coat objects having a complex geometry, then it is problematic, in shadow regions, for example, to produce a high level of chemical and physical resistance in the coating.
WO-A 00/59978 discloses an aqueous coating material which is curable thermally and/or with actinic radiation and comprises a polyurethane containing active hydrogen atoms, based on bis(4-isocyanatocyclohexyl)methane with a trans/trans content of up to 30% by weight, based on the diisocyanate, as binder and at least one crosslinking agent. Unsaturated groups are introduced laterally or terminally into the polyurethane by means of low-molecular-weight, isocyanate-reactive compounds such as trimethylolpropane monoacrylate. A disadvantage here is the use of monomeric unsaturated units, which, as monofunctional units, restrict the molecular weight of the polyurethane or, as pure difunctional molecules, for example trimethylolpropane monoacrylate, are very expensive.
EP-A 0 952 170 describes a coating system which comprises a urethane (meth)acrylate containing free NCO groups, a photoinitiator and one or more aqueous binder containing active hydrogen atoms. The curing of this system combines both crosslinking via the reaction of the isocyanate groups of the active hydrogen atoms of the binder and UV curing via the urethane (meth)acrylates. A disadvantage here is the relatively low UV reactivity, owing to a low density of double bonds, and a poorer initial physical drying before UV curing, owing to the relatively low molecular mass of the urethane methacrylate.